There is an increasing need for mobile high speed communication systems to provide a variety of services such as the Internet, television, photo sharing, and downloading music files. In order to provide such services, a mobile high speed communication system must be able to overcome a variety of difficult operating conditions caused by the environment. Among these operating conditions are multipath signals, inter-symbol interference (ISI), and inter-channel interference (ICI). In mobile high speed communication systems, multipath is interference resulting from radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from terrestrial objects, such as mountains and buildings. In telecommunications, ISI is a form of distortion of a signal in which one symbol interferes with subsequent symbols. ICI is a form of distortion of a signal caused by transmission of signals on adjacent channels that may interfere with one another.
FIG. 1 illustrates a mobile radio channel operating environment 100. The mobile radio channel operating environment 100 may include a base station (BS) 102, a mobile station (MS) 104, various obstacles 106/108/110, and a cluster of notional hexagonal cells 126/130/132/134/136/138/140 overlaying a geographical area 101. Each cell 126/130/132/134/136/138/140 may include a base station operating at its allocated bandwidth to provide adequate radio coverage to its intended users. For example, the base station 102 may operate at an allocated channel transmission bandwidth to provide adequate coverage to the mobile station 104. The base station 102 and the mobile station 104 may communicate via a downlink radio frame 118, and an uplink radio frame 124 respectively. Each radio frame 118/124 may be further divided into sub-frames 120/126 which may include data symbols 122/128. In this mobile radio channel operating environment 100, a signal transmitted from a base station 102 may suffer from the operating conditions mentioned above. For example, multipath signal components 112 may occur as a consequence of reflections, scattering, and diffraction of the transmitted signal by natural and/or man-made objects 106/108/110. At the receiver antenna 114, a multitude of signals may arrive from many different directions with different delays, attenuations, and phases. Generally, the time difference between the arrival moment of the first received multipath component 116 (typically the line of sight component), and the last received multipath component (possibly any of the multipath signal components 112) is called delay spread. The combination of signals with various delays, attenuations, and phases may create distortions such as ISI and ICI in the received signal. The distortion may complicate reception and conversion of the received signal into useful information. For example, delay spread may cause ISI in the useful information (data symbols) contained in the radio frame 124.
Orthogonal Frequency Division Multiplexing (OFDM) is one technique that is being developed for high speed communications that can mitigate delay spread and many other difficult operating conditions. OFDM divides an allocated radio communication channel into a number of orthogonal subchannels of equal bandwidth. Each subchannel is modulated by a unique group of subcarrier signals, whose frequencies are equally and minimally spaced for optimal bandwidth efficiency. The group of subcarrier signals are chosen to be orthogonal, meaning the inner product of any two of the subcarriers equals zero. In this manner, the entire bandwidth allocated to the system is divided into orthogonal subcarriers.
Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user version of OFDM. For a communication device such as the base station 102, multiple access is accomplished by assigning subsets of orthogonal sub-carriers to individual subscriber devices. A subscriber device may be a mobile station 104 with which the base station 102 is communicating.
An inverse fast Fourier transform (IFFT) is often used to form the subcarriers, and the number of orthogonal subcarriers determines the fast Fourier transform (FFT) size (NFFT) to be used. An information symbol (e.g., data symbol) in the frequency domain of the IFFT is transformed into a time domain modulation of the orthogonal subcarriers. The modulation of the orthogonal subcarriers forms an information symbol in the time domain with a duration Tu. Duration Tu is generally referred to as the OFDM useful symbol duration. For the subcarriers to remain orthogonal, the spacing between the orthogonal subcarriers Δf is chosen to be
      1          T      u        ,and vice versa the OFDM symbol duration Tu is
      1          Δ      ⁢                          ⁢      f        .The number of available orthogonal subcarriers NC (an integer less than or equal to NFFT) is the channel transmission bandwidth (BW) divided by the subcarrier spacing
      BW          Δ      ⁢                          ⁢      f        ,or BW*Tu.
FIG. 2 illustrates principles of an OFDM/OFDMA multicarrier transmission with four subcarriers. The principle of multi-carrier transmission is to convert a serial high-rate data stream 202 into multiple parallel low-rate sub-streams 204 by a serial-to-parallel converter. Each parallel sub-stream is modulated on to one of NC orthogonal sub-carriers 206, where NC is an integer that, for example, can be greater than or equal to 128. The NC sub-streams are modulated onto the NC sub-carriers 206 with a spacing of Δf in order to achieve orthogonality between the signals on the NC sub-carriers 206. The resulting NC parallel modulated data symbols 210 are referred to as an OFDM symbol. Since the symbol rate on each sub-carrier 206 is much less than the symbol rate of the initial serial data 202, the OFDM symbols are less sensitive to timing. Thus, the effects of symbol overlap (i.e., ISI) caused by delay spread decrease for the channel.
FIG. 3 illustrates ISI between OFDM/OFDMA symbols. As shown in FIG. 3, OFDM/OFDMA symbols S1-S3 may be transmitted on the sub-frame 120 of the downlink radio sub-frame 118 from the base station (BS) 102 to the mobile station (MS) 104 (FIG. 1). Multipath components 112 (FIG. 1) may cause a delay spread 302 of the symbols S1-S3. The delay spread may cause the OFDM/OFDMA symbols S1-S3 to overlap each other, such that ISI 304 occurs between OFDM/OFDMA symbols S1-S2 and S2-S3. If the ISI is large enough, the signal reception may be disrupted.
In order to make an OFDM/OFDMA system more robust to multipath signals, an extension is made to the information symbol called a cyclic prefix. The cyclic prefix 402 is generally inserted between adjacent OFDM/OFDMA symbols as shown in FIG. 4. The cyclic prefix 402 is typically pre-pended to each OFDM/OFDMA symbol and is used to compensate for the delay spread introduced by the radio channel as explained below. The cyclic prefix 402 can also compensate for other sources of delay spread such as that from pulse shaping filters often used in transmitters. By significantly reducing or avoiding the effects of ISI and ICI, the cyclic prefix 402 also helps to maintain orthogonally between the OFDM/OFDMA signals on the sub-carriers 206 (FIG. 2). The cyclic prefix 402 has a duration TG, which may be added to the useful symbol duration Tu. Thus, a total OFDM/OFDMA symbol duration may be TSYM may be Tu+TG. Although, in this example, a total OFDM/OFDMA symbol duration of TSYM=Tu+TG may be employed for transmitting an OFDM/OFDMA symbol, only the useful symbol duration Tu (FIG. 2) may be available for user's data symbol transmission.
As mentioned above, the cyclic prefix 402 is a cyclic extension of each OFDM/OFDMA symbol, which is obtained by extending the duration of an OFDM/OFDMA symbol. FIG. 5 shows an exemplary cyclic prefix. In FIG. 5, a sinusoidal curve 504 corresponds to an original sinusoid where one cycle of the sinusoid is of duration 3.2 μs (i.e., 64 samples with 20 MHz sampling rate). For this example, the subcarrier frequency is 312.5 KHz. A cyclic prefix 502 of 16 samples (0.8 μs) is pre-appended to the original subcarrier 504 which still has the original sinusoid of frequency 312.5 KHz. The sinusoid is now of duration 4.0 μs, which allows the receiver to choose one period (3.2 μs) of the subcarrier 504 from the bigger window (4.0 μs). In this manner, the cyclic prefix 502 acts as a buffer region. The receiver at the mobile station 104 (FIG. 1) may exclude samples from the cyclic prefix 502/402 that are corrupted by the previous symbol when choosing samples for OFDM/OFDMA symbols (e.g., S1-S3 (FIG. 3)). The cyclic prefix 502/402 duration should be optimized to increase bandwidth efficiency (i.e., bit/Hz).
In telecommunications, a frame is a fixed or variable length packet of data, which has been encoded by a communications protocol for digital transmission. A frame structure is the way a communication channel is divided into frames (e.g., 118/124 in FIG. 1) or sub-frames (e.g., 120/126 in FIG. 1) for transmission. The frame structure of an OFDM or OFDMA system contributes to determining the performance of a communication system. In a communications system, the size and timing of a cyclic prefix in a frame is specified by a frame structure.
In existing OFDM/OFDMA systems, such as Wireless Interoperability for Microwave Access (WiMAX), the cyclic prefix is configurable, but it is fixed when a system is deployed. This limits configuration of the system for efficient bandwidth utilization since the cyclic prefix cannot be reconfigured. Additionally, in existing frame structures, there are no mechanisms to allow a base station to change or reconfigure the cyclic prefix duration for different communication usage scenarios. For example, when communication in a channel suffers from severe multipath effects (i.e., large delay spread), a longer cyclic prefix can be used to eliminate the ISI and ICI. In less severe channel conditions, with fewer multipath issues, a short cyclic prefix can be used in order to reduce overhead and transmission power. Therefore, there is a need for systems and methods that provide a frame structure for high performance OFDM and OFDMA systems that more efficiently use the cyclic prefix.